Process for improving thermal efficiency of gas combustion shale retorting



United States Patent US. Cl. 20811 4 Claims ABSTRACT OF THE DISCLOSUREGas temperature in a shale combustion zone is regulated by passingcombustion air concurrently with downwardly moving shale and removinggaseous combustion products prior to contracting the spent shale withupwardly moving recycle gas. Recycle gas is heated by the pent shalefrom the combustion zone and is directed from the shale cooling zone tothe retorting zone without passing through the combustion zone.

BACKGROUND OF THE INVENTION Field of the invention Oil shale is asedimentary rock which contains a solid organic material known askerogen. When this oil shale is heated to an elevated temperature, thekerogen is decomposed by pyrolysis to shale oil, gas and a carbonaceousresidue commonly identified as coke. A common process for eifectingkerogen pyrolysis is by retorting in a gas combustion shale oil process.A gas combustion oil shale process is carried out in a retort comprisingfour zones known as:

1) A shale preheating zone forming the upper part of the retort whereinraw crushed shale of a granular size is introduced and brought up toretorting temperatures by direct heat exchange with a heat yieldingfluid;

(2) A retorting zone wherein the kerogen component of the shale isdecomposed to shale oil vapors and gas;

(3) A combustion zone wherein controlled combustion of the availablecombustible material is effected to provide at least a portion of theheat energy required in the retorting operation, and

(4) A shale cooling zone wherein the spent shale particles are cooled toa desired low temperature suitable for handling, while preheating atleast a portion of the recycled gases separated from the shaledecomposition products of the retorting operation.

In gas-combustion type retorting processes, crushed raw shale materialhaving a particle size of from about A to about 3 inch particles andhereinafter referred to as granular particle material, is passeddownwardly through the zones comprising the retort as a relatively densemoving bed of granular material while recycle and other gases passedthereto move generally upwardly through the retort and countercurrent tothe downwardly moving shale.

Some advantages of the above-discussed system are attributable to itssimplicity, potentially large capacity and relatively efficient heatutilization. Accordingly, in an acceptable operation, the spent shaleparticles are cooled sufficiently in the retort to exit therefrom at arelatively low temperature, preferably below about 400 F., suitable forhandling and disposal without resorting to expensive heat exchangeequipment. The shale oil exits from the preheating zone in the form of afog or mist after having released its heat to the incoming raw shale.The shale oil mist is separated from the gas mixed therewith, andrecovered.

3,503,869 Patented Mar. 31, 1970 Description of the prior art In priorart oil shale gas combustion processes, gas is passed upwardly through avertical retort and exits from the top portion thereof together withshale oil mist. In the combustion zone, high temperatures develop whichcause shale weakening with resultant shale fines forma tion. The finesare carried upwardly with the gaseous combustion products into theretorting zone. In the retorting and shale preheating zones, the oilformed by kerogen decomposition can be adsorbed on the fines and carrieddownwardly into the combustion zone. In the combustion zone, the shaleoil is partially converted to coke which results in oil yield loss andpossible stoppage of shale flow. In addition, when operating in themanner shown by the prior art, the hot gas from the combustion Zone mayhave such a high temperature as to cause shale oil conversion to coke inthe retorting zone. In addition, too high a combustion gas temperaturecan result in the shale oil product from the retort having too high atemperature. This results in a need for condensing vaporous product topermit recovery of the lower boiling portions of the oil product. Inareas where oil shale is normally found, water is scarce andcondensation as a means to recover oil product is often uneconomical.

In addition, in prior art processes, hot recycle gas from the shalecooling zone is passed directly into the combustion zone. Vaporoushydrocarbons comprise a large portion of this recycle gas and a portionof these hydrocarbons are burned in the combustion zone. Operating thecombustion zone in this manner results in relatively incompletecombustion of the coke on the shale. This is due to the constantintroduction of hydrocarbon recycle gas into the combustion zone. Sincea limited amount of free oxygen is introduced into the combustion zonesufficient to supply the heat requirements of the system, a largeportion of the hydrocarbon vapors will be burned while a correspondingportion of the coke required for heat release will remain on the shale.This is undesirable since spent shale usually leaves the retort at atemperature of from about 300 F. to about 400 F. and upon contact withoutside air the residual coke thereon may burn. The constantintroduction of recycle gas containing shale oil vapor into thecombustion zone causes a loss of shale oil yield and high temperaturesto occur in a relatively small volume of the shale bed. These hightemperatures promote undesirable carbonate decomposition and finesformation.

It is known in the prior art to recover shale'oil from a gas combustionretorting process by withdrawing shale oil vapors from a verticallyintermediate stage in the retort; as for example, US. Patent 2,710,828,issued June 14, 1955. The vapors are withdrawn from the retorting zoneor above the retorting zone and are processed to condense a portion ofthe shale oil therein. The remaining vapors are directed to the shalepreheating zone. However, in these processes, there is no provision forminimizing direct contact between combustion supporting gas, and fineswith kerogen decomposition products and oil yield losses result. In theprior art processes, the means for regulating retorting temperatures areseverely limited and there is no provision for minimizing contact ofcombustion gas with recycle gas.

SUMMARY OF "II-IE INVENTION By the present invention, a modified gascombustion shale retorting process is provided wherein direct contact isminimized between gas combustion products from the combustion zone andboth kerogen decomposition products in the retorting zone and recyclegas from the shale cooling zone. Shale having coke thereon from aretorting zone is passed downwardly to a combustion zone. In thecombustion zone, the shale is contacted with free-oxygen containing gasto promote combustion of the coke and heat the shale and gas. Thecombustion gas and shale in the combustion zone are passed concurrentlydownward through the combustion zone. The shale from the combustion zoneis passed over gas disengagement means located within the retort toseparate gaseous combustion products from the spent shale prior tocontacting recycle gas from the shale cooling zone. The gaseouscombustion products are removed from the retort while the separatedspent shale is passed downwardly through a shale cooling zone. In theshale cooling zone, the spent shale is contacted with upwardly movingrecycle gas whereby heat is transferred from the spent shale to therecycle gas. The recycle gas is removed from the retort prior toentering the combustion zone by gas disengaging means located within theretort. The spent shale is removed from the retort after passing throughthe shale cooling zone. The heated recycle gas can contain fines andafter being removed from the retort is directed to a fines separationzone wherein fines are separated from the heated recycle gas. Therecycle gas from the fines separation zone is directed to the retortingzone located above the combustion zone and is therein passed upwardlythrough the downwardly moving shale. In the retorting zone, kerogen inthe raw shale is thermally decomposed to oil vapors. The resultantgasiform material is passed upwardly through incoming raw shale totransfer heat from the gasiform material to the raw shale. This resultsin the formation of an oil mist or fog which is removed from the shalepreheating zone and directed to an oil separation zone. In the oilseparation zone, liquid oil product is separated from gasiform material.A portion of the gasiform material is recycled to the lower portion ofthe shale cooling zone in a manner described above.

Thus by the process of this invention, transfer of gaseous products fromthe combustion zone to either the retorting zone or the shale coolingzone is minimized. This results in substantial advantages for theoverall gas combustion process. By minimizing contact between gaseouscombustion products and oil formed by kerogen decomposition in theretorting zone, the chances of burning shale oil product are minimized.In addition, because the large proportion of fines found in a retortingprocess are formed in the combustion zone, the chances for contactingfines and shale oil product with the attendant undesirable results areminimized. The process of this invention also provides substantialadvantages by removing the recycle gas from the retort prior tocontacting gaseous products from the combustion zone. In this manner,selective burning of coke on the shale is accomplished while minimizingburning of gaseous hydrocarbons.

This results in more uniform coke burning and prevents substantial lossof shale oil vapors in the combustion zone. Accordingly, higher retortshale oil yields are obtained since condensible oil not previouslyseparated will not be consumed as fuel in the combustion zone.

The present invention also provides substantial advantage in that thevent gas (i.e., excess gas from the oil product separation step overthat needed for recycle gas) will be characterized by a higher heatcontent per unit volume and hence more valuable. This is a result ofseparately removing gaseous combustion products and recycle gas from theretort prior to their being substantially mixed. Thus, the gaseousproducts subsequently recovered from the oil separation step willcontain substantially reduced concentrations of nitrogenous gases andgaseous carbon oxide as compared to the prior art processes. The excessvent gas can be employed as a fuel gas in outside processes. Inaddition, the heat from the gaseous combustion products can be recoveredby heat exchange. Thus one net result of the present invention is thatthe heating potential of the coke produced is recovered for use in theretorting process and carbonaceous gaseous by-product is freed for usein other processes. This promotes a substantially improved thermalefiiciency of the overall process.

The concurrent fiow of combustion supporting gas and shale in thecombustion zone of this invention is desirable over the countercurrentflow of gas in the prior art. In countercurrent flow of prior art it hasbeen found that fines tend to separate out and accumulate in the bedcausing uneven gas flow, shale flow stoppage and poor unit operations.In concurrent flow of the shale and gas in the combustion zone of thisinvention fines will not separate out but will be swept along with thedownwardly flowing gas. Thus the undesirable effect of fines separationand accumulation in the bed above is avoided in this invention.

In the process of this invention, pressures within various portions ofthe retort are maintained at levels to insure desired gas flow. Thepressures at the lower portion of the combustion zone where combustionproducts are removed from the retort and the higher portion of the shalecooling zone where hot recycle gas is removed, are maintainedsubstantially equal to minimize gas flow therebetween, This can beaccomplished by means well known in the art as for example bycontrolling valves at each of the gas outlets, said valves beingresponsive to a pressure differential between the two zones. Thepressure at the upper portion of the combustion zone and the lowerportion of the shale cooling zone at the respective gas inlets aremaintained somewhat above the pressures at the above mentioned gasoutlets to eifect gas flow from the respective gas inlets to the gasoutlets. The pressure in the retorting zone is maintained somewhat belowthe pressure in the upper combustion zone to minimize flow of shale oilproduct to the combustion zone. Flow of combustion gas to the retortingzone is minimized by the use of standpipes for shale flow therebetweenhaving a suflicient vertical height to absorb the difference in pressurebetween the top of the combustion zone and the bottom of the retortingzone while minimizing gas flow from the combustion zone to the retortingzone. The standpipes are at least long enough so the head of solidstherein is greater than the pressure difference between zones.

BRIEF DESCRIPTION OF THE DRAWING The attached figure shows one interiorarrangement for a retort wherein gases in the combustion zone areretained separate from both the retorting zone gas and the shale coolingzone gas.

DESCRIPTION OF SPECIFIC EMBODIMENTS Referring now to the attacheddrawing, crushed raw shale particles having a size of from about A toabout 4 inches mean diameter is introduced into retort 1 through closedconduit 2. The shale particles are directed through a plurality ofconduits 3 to the top of a downwardly moving compact bed of shaleparticles in preheating and retorting zone 4. In this manner, relativelyeven particle distribution over the bed surface area is effected. Thetop of the particle bed in preheating and retorting zone 4 is maintainedat a desired distance below the top of the retort 1 to form a plenumchamber 5 which facilitates separation of particles and oil mist. Inpreheating and retorting zone 4, the incoming particles are heated byupwardly moving vapors and reach retorting temperature as the particlesmove downwardly. The heat supplying gas is introduced through conduit 6into header 7 and is introduced into the bed through distributors 8.When retorting temperature is reached in zone 4, the kerogen decomposesto form gas, oil vapors, and coke residue. The spent shale having cokethereon is passed downwardly through standpipes 9 into combustion zone10. The standpipes 9 extend from baffle 11 to form a plenum chamber 12and to provide back pressure means to prevent incoming combustion gasfrom entering preheating and retorting zone 4. Provision can be made tointroduce seal gas into each standpipe 9.

Combustion gas is introduced into plenum chamber 12 through conduit 13.The combustion gas is passed downwardly from plenum chamber 12 into thecombustion zone to eifect burning of coke on the shale particles. Incombustion zone 10, gaseous material and shale particles are passedconcurrently downwardly to contact a gas-solids disengager comprising aplurality of collectors 14 and a header 15. The gaseous combustionproducts and spent shale fines are removed from the retort throughheader 15 and conduit 16. Due to the burning effected in combustion zone10, the shale particles become heated. The heated shale particles passdownwardly from the combustion zone 10 to shale cooling zone 17 whereincontact is made with relatively cool recycle gas. The recycle gas isintroduced into the retort 1 through conduit 18, header 19 anddistributors 20. The recycle gas passes upwardly through the shale cooling zone 17 to contact downwardly moving shale particles. In thismanner, heat is transferred from the shale particles to the recycle gas.The cool shale particles are removed from retort 1 through conduit 21and discarded. The heated recycle gas is removed from retort 1 by way ofgas-solids disengager comprising header 22 and collectors 23 and conduit24. Means now shown are provided in conduit 21 for effecting relativelyuniform shale flow through the retort 1.

The oil mist or fog separated from the shale preheating zone 4 isremoved from plenum chamber 5 through conduit 25. The oil mist isdirected to a separation step 26 wherein liquid oil product is separatedfrom vaporous material. An electrostatic precipitator and/or cyclone orthe like can be employed in the separation step 26. Oil product isobtained and recovered through conduit 27. Gaseous material is recoveredfrom separation step 26 and recycled in a manner whereby a portion isdirected to the shale cooling zone 17, another portion can be mixed withthe gaseous efliuent from the shale cooling zone, and the remainder isvented to be employed in processses not shown. The gas recycled to theshale cooling zone 17 is pumped by pump 28 through conduits 29 and 18into header 19. The gas to be mixed with hot recycle gas is pumpedthrough conduits 18, 30 and 6 and mixed with hot recycle gas fromconduit 31. Alternately, this cooling gas may pass to conduit 24 throughconduit 32. Excess recycle gas is vented through conduit 33. Hot recyclegas is removed from the retort through conduit 24 and directed to afines separation step 35 wherein fines carried over with the gas areseparated and removed through conduit 36. The hot recycle gas isdirected to the preheating and retorting zone 4 through conduits 31 and6.

Combustion air is introduced into plenum chamber 12 through conduit 13.The air can be mixed with combustion products obtained from the retortand directed through conduits 37 and 38 prior to being introduced intothe retort. This is an effective means for diluting the combustion airto control burning in the combustion zone 10 and minimize thedevelopment of high localized peak temperatures. The gaseous combustionproducts are withdrawn from the retort through conduit 16 and directedto a separation zone 39 wherein carried over fines are separated fromthe gases and removed through conduit 40. The combustion gas exits fromthe retort at relatively high temperatures and the heat therein can beextracted by heat exchange means not shown. The heat from the gaseouscombustion products can \be converted to steam, for example, which canthen be used to drive the compressors which pump recycle gas andcombustion air to and from the retorting process.

The amount of free oxygen which is introduced into the combustion zoneis regulated so as to maintain the 6 shale temperature in the combustionzone between about 900 F. and about 1400 F., preferably between about950 F. and about 1100 F. By operating in this manner, sufficient heatcan be transferred to the recycle gas in the shale cooling zone tosupport kerogen decomposition in the retorting zone without causingexcessive shale disintegration in the combustion zone. The amount offree oxygen in the combustion zone can be regulated by dilutingcombustion air with flue gas to obtain a free-oxygen concentration inthe resultant mixture usually between about 5 and about 15 volumepercent. Since carbon burning rate is proportional to oxygen partialpressure, the diluted gas will minimize the attainment of high peaktemperatures and carbonate decomposition in the combustion zone whilethe combustion gas is being difiused into the bed. The amount of recyclegas directed to the shale cooling zone is sulficient to reduce the shaletemperature to below about 400 F. and preferably below about 300 F. Byoperating in this manner, the hot recycle gas from the fines separationstep associated with the shale cooling step can have a temperature ashigh as about 1400 F.

It therefore may become desirable to reduce the hot recycle gastemperature prior to introducing it into the retorting zone. Thus thepresent invention provides for mixing the hot gas with a portion of thecold recycle gas prior to introducing gas into the retorting zone. Therelative amounts of cold and hot gas mixed is that which will maintainthe maximum retorting temperature between about 700 F. and about 1100F., preferably between about 850 F. and about 1000 F. In this manner,increased retorting times result which increases oil yield. The oil mistexits from the top of the shale retorting and preheating zone at atemperature below about 300 F. and preferably between about F. and about200 F. In the process of the present invention, the hot recycle gas canbe introduced thereto by means of a plurality of inlets at differentvertical heights. This permits more effective heating in the retortingzone and permits heat soaking of the shale in the retorting zone. Thisheat soaking effects increased vaporization of the liquid decompositionproducts and this provides for increased shale oil yields andflowability of the solids.

Having thus presented a general description of the process of thisinvention, it is to be understood that minor modifications can be madethereto without departing from the scope thereof and no unduerestrictions are to be imposed by reason of the specific examplespresented.

We claim:

1. A method for retorting oil shale particulate material which comprises(a) passing oil shale particulate sequentially downwardly through a gascombustion retort comprising a shale preheating-kerogen decompositionretorting zone, a combustion zone and a shale cooling zone,

(b) passing hot recycle gas obtained from the upper end of said shalecooling zone by countercurrent passage therethrough into a fines-removalzone and thence into the lower portion of the shale retorting zone forupward flow therethrough under kerogen decomposition conditions,

(0) effecting decomposition of kerogen in said oil shale particulatewith said hot recycle gas and recovering gasiform products ofdecomposition including gaseous hydrocarbons and liquid hydrocarbonsfrom the upper portion of said preheating zone,

(d) passing an oxygen-containing combustion gas concurrently withretorted oil shale particulate downwardly through said combustion zoneunder conditions to further heat said shale particulate tov an elevatedtemperature by burning combustible materials associated therewith,

(e) removing hot gaseous combustion products from the lower portion ofsaid combustion zone prior to hot shale particulate coming in contactwith preheated recycle gas removed from the upper portion of said shalecooling zone, and

(f) passing recycle gaseous hydrocarbons separated from liquid productsof retorting countercurrent to and upwardly through said shale movingdownwardly through said shale cooling zone from said combustion zone.

2. The method of claim 1 wherein a portion of the recycle gas separatedfrom the liquid oil product is mixed with hot recycle gas from the shalecooling step prior to introducing the hot recycle gas into the retortingzone.

3. The method of claim 1 wherein the gaseous combustion products arefreed of entrained fines and thereafter heat is recovered therefrom as asource of power for pumping recycle gas and combustion gas.

' 4. The method of claim 1 wherein combustion gas freed of entrainedfines is employed to dilute oxygen-conintroduction-into the com-References (lited I UNITED STATES PATENTS 11/1955 Leffer 20811 11/1957Van Dijck 208-11 5/1968- Peet 208-11 FOREIGN PATENTS 10/1960 Canada.

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